1. Field of the Invention
The present invention relates to apparatus and method for diagnosing leaks of a fuel vapor treatment unit of a vehicle internal combustion engine, for diagnosing the presence of leaks in piping.
2. Description of the Related Art
With conventional fuel vapor treatment units for vehicle internal combustion engines, the fuel vapor produced in the fuel tank etc. is temporarily adsorbed in a canister. Then under predetermined engine operating conditions, the adsorbed fuel vapor is de-adsorbed and drawn into the engine intake system, thereby preventing evaporation of the fuel vapor into the atmosphere (refer to Japanese Unexamined Patent Publication No. 5-215020).
With the above unit however, if a crack occurs along the piping, or a fault occurs in a seal at a piping connection, then some of the fuel vapor from the leaking portion will evaporate into the atmosphere, so that the original evaporation prevention effect cannot be fully realized.
Therefore, a leak diagnosis apparatus has been disclosed in Japanese Unexamined Patent Publication No. 7-139439 which confines the negative intake pressure of the internal combustion engine inside the piping and then diagnoses the presence of leaks based on a change in pressure in the piping.
However, with an apparatus for diagnosing the presence of leaks in this manner based on a change in pressure, there is the problem that in the case of minute holes in the piping where the amount of leakage is small, it is difficult to diagnose with high accuracy.